Adaptive control unit with feedback compensation

ABSTRACT

An adaptive control unit is described for receiving an analog input signal containing at least an indication of a parameter to be controlled to generate an analog output signal for control of the parameter. The analog input signal contains a fed back component resulting from the analog output signal. The adaptive control unit comprises an analog filter having an adjustable gain, a gain adjuster for adjusting the gain of the analog filter using a feedforward adjustment method, and a filter for compensating for the fed back component in the analog input signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International ApplicationPCT/GB2004/002077 with an International Filing Date of May 14, 2004, andclaiming priority to co-pending Great Britain Patent Application No.0311085.5 filed on May 14, 2003, both of which are incorporated hereinby reference.

The present invention relates to an adaptive control unit with feedbackcompensation.

Active control systems for controlling a parameter in a physical system(a plant) detect the parameter to be controlled and generate a controlsignal. In order to adapt control behaviour, a residual parameter isdetected, i.e. an error detection is made, and this is used to modifythe control behaviour in order to achieve better control. One problem insuch control systems is the coupling that occurs between the output ofthe control system and the input of this control system i.e. undesirablefeedback or pollution of the reference signal.

In one class of control systems, termed virtual earth feedback systems,the input to the control system, i.e. the measurement of the parameter,is made close to the point of control. The system thus acts in afeedback manner to drive the parameter to a desirable value such as tozero when the parameter is undesirable. However, in such systemsstability of the feedback loop is a problem since the detections madeare detections both of the undesired parameter and feedback from theoutput of the control system.

In another class of control systems, termed feed forward controlsystems, a reference signal is obtained which is indicative of theparameter to be controlled. This reference signal provides an indicationof the degree of success of the control system in achieving the desiredcontrol. In a perfect feed forward control system, the reference signalis unpolluted by the control output of the control system. However, inpractice often the reference signal is polluted by the output controlsignal. This can present difficulties in achieving stable control.

Control systems can be analog, digital, or a hybrid of analog anddigital. For example, GB 2142091, the content of which is herebyincorporated by reference, discloses a feed forward control system forthe attenuation of sound. In this system an analog feed forwardcontroller is provided with a digital feedback compensation filter tocompensate for coupling between the sound generating and detectionsystems. In this system the gain of the analog amplifier is fixed. Thus,this system is highly limited and does not provide for adaptive control.

GB 2222733, the content of which is hereby incorporated by reference,describes a hybrid analog and digital filter. The system disclosedovercomes the problem of the latency of digital controllers by providingan analog bypass filter in parallel with the digital filter. Digitalfilters have a latency problem due to analog to digital conversiondelays, processing delays, and digital to analog conversion delays. In awholly digital system this problem can only be overcome by increasingthe sample rate, i.e. processing speed, thus increasing complexity andcost of the digital system. The provision of the analog bypass filter inparallel to the digital filter overcomes the latency problem of thedigital controller. However, this document does not disclose any methodof adaption of the hybrid analog and digital filter.

U.S. Pat. No. 6,278,786, the content of which is hereby incorporated byreference, discloses a hybrid analog and digital control system for anactive noise cancellation headset system. The system is illustratedschematically in FIG. 1. A microphone 1 and loudspeaker 6 are mounted inan earcup of a headset. Thus because of the close proximity of themicrophone 1 and the loudspeaker 6, the system comprises a virtual earthsystem. The output of the microphone 1 is digitised by an analog todigital converter 2 and the digitized signal is input to a digitalsignal processor 3. The digital signal processor includes a digitalfilter for generating a digital filtered signal for conversion to ananalog control signal by a digital to analog converter. The input fromthe microphone 1 is also input through a compensation filter 7 andthrough an amplifier 8 having a gain G to generate an analog controlsignal. The analog control signal from the amplifier 8 and the analogcontrol signal from digital to analog converter 4 are summed in asumming amplifier 5 before being output to the loudspeaker 6. Thus, thesystem comprises a digital control path in parallel with an analogcontrol path. The digital signal processor 3 also controls the gain ofthe amplifier 8. The method disclosed for control is to increase theloop gain until the system is on the verge of instability, thusobtaining maximum noise reduction on the non-adaptive analog componentof the system under all conditions. Thus this system does not provide afully adaptive analog gain control loop. Further, this system makes noprovision for pollution of the input signal by feedback coupling withthe output signal.

In digital control systems a well known feed forward control system isthe filtered X LMS algorithm. This is illustrated in FIG. 2 andreference to it can be found in “Adaptive Signal Processing” by BernardWidrow and Samuel B. Stearns, the content of which is herebyincorporated by reference. A reference signal is detected in an activevibration system using a vibration sensor such as a microphone 10. Theinput signal is digitised using an analog to digital converter. In thisexample a feedback signal is subtracted from the input signal using asubtractor 12. A feedback signal is obtained by filtering the output ofthe adaptive filter 13 through a plant model Ĉ. Thus in this example,pollution of the input reference signal by the output signal isaccounted for. Thus the reconstructed (unpolluted) references input intothe adaptive filter 13 and the output of the adapted filter is input toa digital to analog converter 14 for output to a loudspeaker 15 fornoise cancellation.

In order to perform adaption of the filter characteristics of the filterW 13, the reconstructed reference is input through a model of the plantĈ 18 to provide a time aligned reference r which is input to an LMSalgorithm 17 together with the input error signal e from the analog todigital converter 11. The LMS algorithm 17 determines updatedcoefficients for the adaptive filter W 13.

Thus in the example given in FIG. 2, although a reconstructed referenceis provided in a feed forward control system using a model Ĉ of theplant, i.e. the acoustic response of the loudspeaker, the acousticenvironment, and the microphone 10, the system suffers from the latencyproblem of a digital control system.

It is thus an object of the present invention to provide an improvedadaptive control system in which compensation for the feedback from theoutput of the control system to the input of the control system isprovided for in an adaptive analog or hybrid analog and digital controlsystem.

One aspect of the present invention provides an adaptive control systemfor receiving an analog input signal containing at least an indicationof the parameter to be controlled to generate an analog output signalfor control of the parameter in which the analog input signal contains afed back component resulting from the analog output signal. The adaptivecontrol unit comprises an analog filter having an adjustable gain, gainadjusting means for adjusting the gain of the analog filter using thefeed forward adjustment method, and filtering means for compensating forthe fed back component in the analog input signal.

Thus this aspect of the present invention provides for the compensationfor feedback from the output signal in the input signal thus providing areconstructed reference signal which, even in a virtual eartharrangement enables a feed forward control method to be used on thebasis of the reconstructed reference. This facilitates the adaptivecontrol of the gain of the analog filter thus providing a more usefuladaptive analog control system.

In one embodiment the gain adjustment is determined using an indicationof the parameter in the analog input signal and an error component. Inone embodiment the analog input signal is used for the gain adjustmentafter filtering by the filtering means. In a virtual earth feedbackembodiment the error component is obtained from the analog input signal.In a feed forward embodiment the error component is obtained from aseparate error signal.

Because of the reconstruction of the reference signal by the removal ofthe coupling between the output signal and the input signal of theadaptive control unit, as mentioned above, the control problem for theanalog filter becomes a feed forward control problem and thus well knownfeed forward adaptive algorithms can be used. One such family ofalgorithms is the filtered reference or filtered error signal method. Inparticular the filtered X LMS algorithm or variants thereof can be used.Examples of such types of algorithms are the Newton method, and thefiltered error LMS algorithm. Such algorithms are well known to askilled person in the art.

In one embodiment of the present invention an analog subtractor meanssuch as a subtracting amplifier is provided for subtracting an output ofthe filtering means from the analog input signal before input into theanalog filter. Thus in this embodiment the compensation of the backelement, i.e. the reconstruction of the reference, is provided by analogsubtraction of the feedback component from the input polluted referencesignal. Because this compensation takes place in the analog domain,there is no latency problem which is associated with such a compensationtechnique in digital adaptive control systems.

In one embodiment of the present invention filtering means is adapted tofilter the analog input signal using a model of at least a phaseresponse of a feedback path of the analog output signal to the analoginput signal. In this embodiment the model particularly models the plantbeing controlled. For example, in an active vibration control system themodel will comprise a model of the response of the vibration actuator,the acoustic response of the plant, and the response of the vibrationdetector.

In another embodiment of the present invention, the filtering means isadapted to filter the analog input signal using a model of at least aphase response of a feedback path of the analog output signal to theanalog input signal, and a model of at least a phase response of theanalog filter. In the present invention the model can be of the phaseresponse of a plant (path) or the phase and amplitude response of theplant.

In one embodiment the analog subtractor means is arranged forsubtraction of the output of the filtering means from the analog inputsignal before it is input to the filtering means.

The filtering means in accordance with the present invention cancomprise an analog or digital filter. The benefit of an analog filter isthat there is no problem associated with delays caused by analog todigital conversion and digital to analog conversion. However, since thefiltering means in accordance with an embodiment of the presentinvention models the response of the plant, this response can requireadaption or adaptive learning, which presents problems for analogfilters. The benefit of using digital filters is the ease with which thedigital filter characteristics can be adapted to model the plant asrequired.

In one embodiment of the present invention the filtering means comprisesa digital filter and a digital subtractor means is provided fordigitally subtracting a digital representation of the analog outputsignal from a digital representation of the analog input signal. In oneembodiment the digital filter is adapted to filter using a model of atleast a phase response of a feedback path of the analog output signal tothe analog input signal and to include a factor in the modelcompensating for a delay caused by analog to digital conversion anddigital to analog conversion of the digital result of digitallyfiltering.

In one embodiment in which the filtering means is digitally implemented,the filtering means includes an analog to digital converter forreceiving and digitizing the analog input signal, at least one digitalfilter for filtering the digitized input signal and a digital to analogconverter for converting the filtered digitized input signal to producean analog compensation signal for compensating for feedback of theanalog output signal in the analog signal.

In one embodiment the filtering means is adapted to filter the analogoutput signal. In one specific embodiment the filtering means is adaptedto filter using a model of at least a phase response of a feedback pathof the analog output signal to the analog input signal. In one specificembodiment analog subtractor means such as a subtracting amplifier isprovided for subtracting an output of the filtering means from theanalog input signal before input to the analog filter.

Thus this embodiment of the present invention provides for feedbackcompensation by taking the output signal of the analog amplifier. Thefilter means can be analog or digital. If a digital filter is used toprovide for ease of setting up and adaption of the coefficients of thefilter, the digital filter will need to include a factor in the modelcompensating for the delay caused by analog to digital conversion of theanalog output signal and the digital to analog conversion of a digitalresult of digitally filtering.

In one embodiment the gain adjusting means comprises a digital controlmeans. The digital control means can comprise a filtered referencecontrol means. In one embodiment the analog filter comprises anamplifier. The amplifier can comprise a digitally controlled amplifierin one specific embodiment.

In one embodiment of the present invention the analog filter comprisesan analog compensation filter. Thus in this embodiment of the presentinvention the adaptive control unit comprises an adaptive analog filter.

In one embodiment the filtering means and the gain adjusting meanscomprises a programmed digital controller.

The filtering means of the present invention can perform the filteringin the time or frequency domain.

In one embodiment the filtering means comprises a one-bit analog todigital converter for converting the analog signal to a one-bit digitalsignal, a digital filter comprising a model of the feedback path of theanalog output signal to the analog input signal and adapted to digitallyfilter the one-bit digital signal by a series of additions, and aone-bit digital signal to analog converter for converting the output ofthe digital filter to an analog filter output for use in thecompensation for the fed back component in the analog input signal. Thusin this embodiment of the present invention the use of the one-bit ADCand DAC provides for a very simple implementation of the digitalfiltering.

In one embodiment of the present invention the adaptive control unitincludes a digital filter for digitally filtering the analog inputsignal arranged in parallel with the analog filter, and a combiner isprovided for combining the output of the analog filter and the output ofthe digital filter to provide the analog output signal for the controlunit. Thus this embodiment of the present invention provides a hybridanalog and digital control unit in which, due to the reference signalreconstruction, adaption of the gain of the analog filter is providedfor.

In a preferred embodiment the digital filter unit comprises an adaptivedigital filter which can, for example, use a filtered reference methodfor adaption. In a preferred embodiment the adaptive digital filterincludes feedback compensation for compensating for a fed back componentfrom an output of the digital adaptive filter in the analog inputsignal. Thus in this way this embodiment of the present inventionprovides for reconstruction of the input reference signal to compensatefor feedback from the component of the analog control and the componentof the digital control for the hybrid analog-digital control system.

One embodiment of the present invention provides for a multi-channelcontrol system in which the adaptive control unit receives a pluralityof the analog input signals each containing an indication of theparameter to be controlled to generate a plurality of analog outputsignals for control of the parameter. Each analog output signal containsa fed back component resulting from each said analog output signal. Theadaptive control unit comprises a plurality of analog filters, eachhaving an adjustable gain, wherein the gain of each analog filter isadjusted using a feed forward adjustment method and the filtering meanscompensates for the fed back component in each analog input signal.

In one specific embodiment the gain adjustment is carried out using amulti-channel filtered reference method for the adjustment of the gainof each analog filter. For example, the methods can comprise any of thewell known multi-channel feed forward filtered reference methods such asthe filtered X LMS algorithm.

One aspect of the present invention provides a control system forcontrolling a parameter of a plant in which the control system comprisesthe adaptive control unit, plant parameter detection means for detectingthe parameter and for providing the detections as the analog inputsignal to the adaptive control unit, plant control means for receivingthe analog output signal and for controlling the parameter using theanalog output signal.

In one embodiment the plant parameter detection means is adapted to alsoprovide a detected error component in the analog input signal and thegain adjustment means is adapted to determine the gain adjustment usingthe indication of the parameter and the error component. First thisembodiment comprises a virtual earth feedback system.

In an alternative feed forward control system, error detection means areprovided for detecting an error component. Gain adjustment means isadapted to determine the gain adjustment using the indication of theparameter and the error component.

In one embodiment of the present invention the control system comprisesan active vibration control system for controlling vibrations. In thisembodiment the plant parameter detection means comprises at least onevibration sensor, the plant control means comprises at least onevibration actuator and the error detection means comprises at least onevibration sensor.

The present invention is applicable to a proportional integraldifferential (PID) controller in which a proportional unit comprises acontrol unit for adjusting the gain of the proportional unit, anintegral unit comprises a control unit for adjusting the gain of theintegral unit, and a differential unit comprises a control unit foradjusting the gain of the differential unit. Summing means is providedfor summing the output of the proportional unit, the integral unit andthe differential unit to generate an output of the PID controller.

Embodiments of the present invention will now be described withreference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a prior art hybrid analog and digitalvirtual earth feedback control system;

FIG. 2 is a schematic diagram of a prior art digital virtual earthfeedback control system with reference regeneration and using thefiltered X algorithm;

FIG. 3 is a schematic diagram of a virtual earth feedback analogadaptive control unit in accordance with a first embodiment of thepresent invention;

FIG. 4 is a schematic diagram of a virtual earth feedback hybrid analogand digital adaptive control system in accordance with a secondembodiment of the present invention;

FIG. 5 is a schematic diagram of a virtual earth feedback analogadaptive control system in accordance with a third embodiment of thepresent invention;

FIG. 6 is a schematic diagram of a virtual earth feedback hybrid analogand digital adaptive control system in accordance with a fourthembodiment of the present invention;

FIG. 7 is a schematic diagram of a feed forward analog adaptive controlsystem in accordance with a fifth embodiment of the present invention;

FIG. 8 is a schematic diagram of a feed forward hybrid analog anddigital adaptive control system in accordance with a sixth embodiment ofthe present invention;

FIG. 9 is a schematic diagram of a feed forward analog adaptive controlsystem in accordance with a seventh embodiment of the present invention;

FIG. 10 is a schematic diagram of a feed forward hybrid analog anddigital adaptive control system in accordance with an eighth embodimentof the present invention;

FIG. 11 is a schematic diagram of a virtual earth feedback hybrid analogand digital adaptive control system in accordance with a ninthembodiment of the present invention;

FIG. 12 is a schematic diagram of a feed forward hybrid analog anddigital adaptive control system in accordance with a tenth embodiment ofthe present invention;

FIG. 13 is a schematic diagram of a virtual earth feedback analogadaptive control system in accordance with an eleventh embodiment of thepresent invention; and

FIG. 14 is a schematic diagram of a PID controller incorporating threeof the feed forward analog adaptive controllers of the fifth embodimentof the present invention.

In the following embodiments of the present invention, the embodimentsillustrate the application of the adaptive control unit to activevibration control, and specifically to active noise control. However,the present invention is not limited to the application of the adaptivecontrol units to active noise control or active vibration control, andthe present invention is applicable to the active control of anyparameters for a plant i.e. any physical parameters. The parameters cancomprise electrical, acoustic, mechanical, optical, or thermalparameters.

In the following amendments, like reference numerals are used in theembodiments for like components.

The first embodiment of the present invention will now be described withreference to FIG. 3. This embodiment of the present invention comprisesa virtual earth feedback control system in which the gain of an analogcompensation filter is adaptively controlled by compensating forcoupling between the output and the input i.e. feedback in order toregenerate the reference to enable the adaptive control of the gain ofthe compensation filter using a feed forward control method.

Referring to FIG. 3, a microphone 20 detects noise in a region in whicha loudspeaker 29 is positioned to control the noise. For example, themicrophone 20 in the loudspeaker 29 can be provided in a headset. Thesignal from the microphone 20 is an analog signal which is input to ananalog subtractor 26 for subtraction of the feedback signal toregenerate the analog reference signal X for input into the amplifier 27having a variable gain. The output of the amplifier is output throughthe compensation filter 28 to drive the loudspeaker 29.

The signal from the microphone 20 is also input to an analog to digitalconverter 21 which introduces a delay (Z^(−n)) i.e. a sampling delay.The digitised signal (e) is then subject to digital subtraction by asubtractor 22 which performs a digital compensation for the feedbacksignal in order to reconstruct the reference signal (x). Thereconstructed reference signal (x) is then input into a time advancedmodel of the acoustic response on the system. Ĉ is a digital filterwhich models the acoustic response of the system i.e. the response ofthe loudspeaker 29, the acoustic response of the path between theloudspeaker and the microphone, and the response to the microphone 20.The factor Z_((m+n)) comprises a time advancement applied to theacoustic model Ĉ to compensate for the digitisation delay n and thedigital to analog conversion delay m. The output of the filter 23 isthen input into a model 24 of the response of the variable gainamplifier 27. The output is an estimate of the feedback coupling fromthe output of the variable gain amplifier 27 to the digital error signale output from the analog to digital converter 21. This signal is fedback to the subtractor 22 for digital subtraction i.e. for compensationdigitally for the coupling of the output of the control unit to theinput. The output from the model 24 is also input to a digital to analogconverter 25 which incurs the digital conversion delay of n to output ananalog signal to the analog subtractor 26 to subtract the feedbacksignal from the input signal E to regenerate the analog reference X forinput to the variable gain amplifier 27.

In this embodiment of the present invention the reconstructed referencex is digitally filtered not only by a time advanced model of theacoustic response to the system, but also by model of the response ofthe variable gain amplifier because a measurement of the feedback isobtained from the input signal from the microphone 20.

The output of the filter 23 is also input into a delay Z^(−n) 30. Thedelay 30 is provided to reinsert the delay caused by analog to digitalconversion 21. For generation of the reference signal r for use in theLMS algorithm 31. The LMS algorithm 31 also receives the digitisedsignal e from the analog to digital converter 21 to determine adaptivelythe gain of the variable gain amplifier 27. The LMS (least mean squared)algorithm is well-known in the prior art and is one example of a feedforward adapted algorithm for adjustment of the gain of the amplifier 27made possible by the reconstruction of the analog reference X and thedigital reference x.

A second embodiment of the present invention will now be described withreference to FIG. 4. This embodiment of the present invention comprisesa hybrid analog and digital virtual earth feedback control unit. Theadapted control unit of this embodiment of the present inventioncomprises a combination of the virtual earth feedback analog filter ofthe first embodiment of the present invention illustrated in FIG. 3 andthe filtered X LMS digital virtual earth feedback control unit of theprior art illustrated in FIG. 2.

Referring to FIG. 4, a microphone 20 detects noise in a region adjacentto a loudspeaker 29 used for generating the control sound. In thisembodiment, the microphone 20 in the loudspeaker 29 are closely coupledand thus the microphone 20 provides both the reference signal indicatingthe undesirable noise to be cancelled, and the residual noise incombination. The analog output of the microphone 20 E is input to ananalog subtractor 26 for the subtraction of an analog compensationsignal to regenerate the analog reference X. Regenerated reference X isthen input into an amplifier 27 with variable gain. The analog output ofthe amplifier 27 is then input to an analog summation unit 34 forsummation with the output of a digital filter component of the unit. Thecombined output from the analog combiner 34 is then output to theloudspeaker 29.

The output of the microphone 20 is also input to an analog to digitalconverter 21 which experiences the sampling delay of Z^(−n). Thedigitised signal e is then input into a digital subtractor 22 for thesubtraction of the feedback signal. The reconstructed reference signalx′ is then input into the time advanced model of the acoustic responseof the system 23 (ĈZ^((m+n))). The output of the filter 23 is input to asecond filter 24 which a model of the response of the variable gainamplifier 27. The output of the filter 24 is then input to a combiner 38for digital combination with a feedback signal for the digital filterpart. The output of filter 24 comprises the feedback part for the analogcomponent. The output of the combiner 38 comprises the complete digitalfeedback signal for reconstruction of the reference signal to enable forfeed forward control of both the gain in the analog path and theadaptive filter 32 in the digital path. The signal is feed back to thedigital subtractor 22 for the regeneration of the digital reference x′and it is also output through a digital to analog converter 25 forsubtraction by the analog subtractor 26 for the generation of theregenerated analog reference X for input to the variable gain amplifier27.

The output of the filter 23 is also input to a delay unit 30 for delayby Z^(−n). The delay unit 30 adds a delay caused by the analog todigital conversion by the ADC 21 to generate the reference signal r foruse by the LMS algorithm 31. The LMS algorithm 31 also receives thedigital error signal e output from the ADC 21 for performing the LMSalgorithm to determine the gain of the variable gain amplifier 27.

The reconstructed digital reference x′ output from the digitalsubtractor 22 is also input to a digital adaptive filter W 32 for thegeneration of the digital control signal. The digital control signal isdigital to analog converted in DAC 33. The output of the DAC 33 iscombined in the analog combiner 34. The output of the digital adaptivefilter 32 is also feedback through a model of the acoustic response tothe system Ĉ 37 and the output of the filter 37 is combined by thecombiner 38 with output of the filter 24 to generate the combinedfeedback signal. The adaption of the digital filter W32 is performed byinputting the reconstructed reference x′ through a model of the acousticsystem Ĉ 35 to generate a reference r′. The LMS algorithm 36 receivesthe reference r′ and the output e of the ADC 21 to determinecoefficients for the adaption of the filter W 32.

In this embodiment of the present invention, the gain of the variablegain amplifier 27 and the coefficients of the digital filter W 32 areboth adaptively determined by respective LMS algorithms. This feedforward type adjustment of the hybrid analog and digital control is madepossible by the generation of the analog reconstructed reference X andthe digital reconstructed reference x′.

The third embodiment of the present invention will be described withreference to FIG. 5. This embodiment of the present invention comprisesa virtual earth feedback analog adaptive control unit and is similar tothe first embodiment of the present invention except that instead ofperforming compensation using an analog subtractor 26 and a digitalsubtractor 22, a single subtractor 26 a is used and the output of thesubtractor 26 a is the reconstructed analog reference signal X which isinput to the amplifier 27 and the analog to digital converter 21.

This embodiment to the present invention then only differs from thefirst embodiment of the present invention in that the error signal e foruse by the LMS algorithm 31 requires reconstruction. This achieved bythe feedback signal using a digital combiner 39. Thus in this embodimentalthough only one analog subtractor is required instead of an analogsubtractor and a digital subtractor, a digital combiner 39 isadditionally required to reconstruct the error signal e.

The fourth embodiment of the present invention will now be describedwith reference to FIG. 6. This embodiment of the present inventioncomprises a virtual earth feedback hybrid analog and digital adaptivecontrol unit. This embodiment comprises a combination of the thirdembodiment of the present invention with the prior art filter LMSalgorithm illustrated in FIG. 2. This embodiment is similar to thesecond embodiment of the present invention except that the digitalsubtractor 22 is not used and the analog subtractor 26 is arranged asanalog subtractor 26 a so that its output i.e. the reconstructed analogreference X is input into the analog digital converter 21 as describedhereinabove with reference to FIG. 5 for the third embodiment of thepresent invention.

This embodiment of the present invention differs from the secondembodiment of the present invention in that a digital combiner 39 isrequired in order to reconstruct the error signal e for both LMSalgorithms 31 and 36 for the adaption of the analog gain for theamplifier 27 and the filter coefficients for the adaptive filter W 32respectively.

A fifth embodiment of the present invention will now be described withreference to FIG. 7. This embodiment of the present invention comprisesa feed forward analog adaptive control unit. This embodiment of thepresent invention is similar to the first embodiment of the presentinvention except that an error sensing microphone 41 is provided for thedetection of residual vibrations in an area adjacent to the loudspeaker29. The output of the error sensing microphone 41 is input to an analogto digital converter 40 for generation of the digital error signal e forinput into the LMS algorithm 31.

In this feed forward arrangement, the microphone 20 acts as a feedforward microphone detecting undesirable vibrations. This signal is usedfor the generation of canceling vibrations by the loudspeaker 29. Themicrophone 41 detects the error in the noise cancellation for use in theadaption of the gain of the amplifier 27 using an LMS algorithm.Coupling between the loudspeaker 29 and the reference microphone 20 becompensated for using the filters 23 and 24 to enable adaption of thegain of the amplifier 27 using the feed forward method i.e. LMSalgorithm.

A sixth embodiment of the present invention will be now described withreference to FIG. 8. This embodiment of the present invention is a feedforward hybrid analog and digital adaptive control unit. This embodimentcomprises a combination of the fifth embodiment of the present inventionand the prior art filtered X LMS algorithm illustrated in FIG. 2. Thisembodiment of the present invention is similar to the second embodimentof the present invention except that an error sensing microphone 41 isprovided adjacent to the loudspeaker 29 and the microphone 20 isprovided as a reference sensing microphone a distance away from theloudspeaker 29. The output of the error sensing microphone 41 is analogto digital converted by the ADC 40 and the output digital error signal eis input to the LMS algorithm 31 and the LMS 36 for the adaption of thegain of the amplifier 27 and the adaption of the filter coefficients ofthe digital adaptive filter W 32 respectively.

In this embodiment of the present invention any pollution of thereference signal from the microphone 20 by the output of the loudspeaker29 is compensated for by the filtering carried out by the filters 23, 24and 37 to provide a combined feedback compensation signal for digitalcompensation using the digital subtractor 22 and for analog compensationusing the analog subtractor 26.

A seventh embodiment of the present invention will now be described withreference to FIG. 9. This embodiment of the present invention comprisesa feedforward analog adaptive control unit. This embodiment of thepresent invention is similar to the third embodiment of the presentinvention except that an error sensing microphone 41 is provided in thevicinity of the loudspeaker 29. The microphone 20 is provided as areference microphone some distance from the loudspeaker 29. The outputof the error sensing microphone 41 is input to an analog-to-digitalconverter 40 to generate the digital error signal e for input to the LMSalgorithm 31.

Once again, in this embodiment undesirable coupling between the outputof the loudspeaker 29 and the reference microphone 20 resulting inpollution of the reference signal is compensated for by generation of acompensation signal using the filters 23 and 24 and subtraction usingthe analog subtractor 26 a.

An eighth embodiment of the present invention will now be described withreference to FIG. 10. This embodiment comprises a feedforward hybridanalog and digital adaptive control unit. This embodiment comprises acombination of the seventh embodiment of the present invention and thefiltered X LMS algorithm illustrated in FIG. 2. This embodiment is alsosimilar to the fourth embodiment of the present invention except that anerror microphone 41 is provided in the vicinity of the loudspeaker 29 toprovide a measure of the error in the cancellation of sound. Themicrophone 20 acts as a reference microphone. The output of themicrophone 41 is input to analog-to-digital converter 40 to generate thedigital error signal e for input into the LMS algorithm 31 and the LMSalgorithm 36 for the adaption of the gain of the amplifier 27 and thefilter coefficients of the adaptive filter W 32 respectively.

A ninth embodiment of the present invention will now be described withreference to FIG. 11. This embodiment comprises a virtual earth feedbackhybrid adaptive control unit. This embodiment of the present inventionis similar to the second embodiment of the present invention except thatthe filter used for the compensation for the feedback for the digitalfilter components is not Ĉ as in the second embodiment but is insteadZ^(−(m+n))/G. A filter 37 a generates a feedback component which iscombined in a digital combiner 38 a for being input into a combinedfilter unit 23 a which performs a filter operation GĈZ^((m+n)). Theoutput of the filter 23 a comprises the feedback component which isdigital-to-analog converted by the DAC 25 for output to the analogsubtractor 26 and also output to the digital subtractor 22.

In order to generate the reference signal r′ the reference signal x′ ispassed through a filter unit 30 a which filters the reconstructedreference signals x′ by a time advanced model of the acoustic system,i.e. ĈZ^(m). The LMS algorithm 31 receives the reference signal r′ fromthe filter 30 a and the error signal e from the output of the ADC 21.

A tenth embodiment of the present invention will now be described withreference to FIG. 12. This embodiment of the present invention is afeedforward hybrid digital and analog adaptive control unit. Thisembodiment of the present invention is similar to the ninth embodimentof the present invention except that an error sensing microphone 41 isprovided in the vicinity of the loudspeaker 29. The output of the errorsensing microphone 41 is input to an analog-to-digital converter 40 andthe digitized output signal e is input to the LMS algorithm 31 with theLMS algorithm 36.

An eleventh embodiment of the present invention will now be describedwith reference to FIG. 13. This embodiment of the present inventioncomprises a virtual earth feedback analog adaptive control unit.

In this embodiment of the present invention, the compensation isprovided for by using the output of the amplifier 27 and feeding it backto the analog subtractor 26 and the digital subtractor 22. In order todo this an additional analog-to-digital converter 42 is required todigitize the output of the amplifier 27. The output of theanalog-to-digital converter 46 is input into a digital filter comprisinga time advanced model of the acoustic system, i.e. ĈZ^(m). The timeadvancement by Z^(m) is required in order to compensation for the delaycaused by the digital-to-analog converter 25 in order to provide theanalog compensation signal used by the analog subtractor 26. The outputof the filter 43 is also fed back to the digital subtractor 22 for thegeneration of the regenerated reference x. The output of the digitalsubtractor 22 is input to a filter 30 a comprising a time advanced modelof the acoustic response of the system, i.e. ĈZ^(m) in order to generatethe reference signal r for input to the LMS algorithm 31.

This embodiment of the present invention is similar to the firstembodiment of the present invention except that the compensation signalis generated using the output of the amplifier 27 rather than the outputof the ADC 21, i.e. the input to the adaptive control unit. This conceptof using the output of the amplifier 27 to generate the compensationsignal or subtraction by the analog subtractor 26, the digitalsubtractor 22, and the combined analog subtractor 26 a is applicable toany of the previous embodiments described hereinabove. The benefit ofthis embodiment is that no model of the amplifier 27 is required.However, an additional analog-to-digital converter 42 is required inorder to digitize the output of the amplifier 27.

A twelfth embodiment of the present invention will now be described withreference to FIG. 14. This embodiment of the present invention comprisesa proportional, integral, differential (PID) controller comprising threecontrol units of the embodiment of FIG. 7.

In this embodiment the parameter detector 900 detects the parameter tobe controlled. The analog input is input into three subtractors 260, 261and 262. The analog inputs also input to an analog-to-digital converter210 and the output of the analog-to-digital converter 210 is input intorespective controllers 600, 601 and 602 for control of the respectivecomponents DIP. Each controller 600, 601 and 602 comprises thecomponents 22, 23, 24, 30 and 31 illustrated in FIG. 7. The compensationsignal generated by each controller is output to a respectivedigital-to-analog converter 250, 251 and 252 and a respective analogcompensation signal is subtracted by respective subtractors 260, 261 and262 to generate a regenerated reference for input to a respective gaincontrol unit 270, 271 and 272 for control of the gain for D, I and Pcomponents respectively. The output of the variable gain components 270,271 and 272 are input to a differential unit 280 and integral unit 281and a proportional unit 282 respectively. The output of these units isthen combined in a combiner 500 and a combined output is output to aparameter actuator 700 for controller the parameter to be controlled. Anerror sensor 800 is provided to sense the error in the control andanalog-to-digital converter 400 digitizes the output of the sensor 800to provide an error feedback to the respective controllers 600, 601 and602.

Thus in accordance with this embodiment of the present invention, eachof the P, I and D components of a PID controller can be typicallycontrolled using a feedforward technique. This is facilitated by thecompensation for pollution of the reference signal from the referencesensor 900.

Although in embodiment 12 it is illustrated that the control units ofthe seventh embodiment are used for control of each of the components P,I and D, any analog feedforward control unit with feedback compensationcan be used for control of the gain of each of the PID components. Forexample, either the fifth embodiment illustrated in FIG. 7 or theseventh embodiment illustrated in FIG. 9 can be used.

In any of the embodiments described hereinabove the control can beperformed by any suitable digital control device such as a programmablelogic device, or a dedicated hardware device.

The model Ĉ of the plant can be adaptively learnt using known techniquessuch as by the injection of white noise into the system and the adaptivelearning of the adaptive filter coefficients for Ĉ.

The present invention is applicable to the control of any parameters andis not restricted to the control of acoustic vibrations. For example,the present invention is applicable to the control of any physicalparameters such as acoustic, optical, electrical, thermal or magneticparameter and thus the microphones 20,41, and 200 can comprise anysuitable parameter sensor and the loudspeaker 29 and 290 can compriseany suitable parameter actuator. Where the parameter is electrical, noparameter sensor or actuator may be required.

Although the present invention has been described hereinabove withreference to specific embodiments, it will be apparent to a skilledperson in the art that modifications lie within the spirit and scope ofthe present invention.

1. An adaptive control unit for receiving an analog input signalcontaining at least an indication of a parameter to be controlled togenerate an analog output signal for control of the parameter, whereinsaid analog input signal contains a fed back component resulting fromsaid analog output signal, the adaptive control unit comprising: ananalog filter having an adjustable gain; gain adjusting means foradjusting the gain of said analog filter; and filtering means forcompensating for said fed back component in said analog input signal;wherein said gain adjustment means is adapted to determine a gainadjustment using a feed forward adjustment method.
 2. An adaptivecontrol unit according to claim 1, wherein said gain adjustment means isadapted to determine said gain adjustment using said indication of saidparameter in said analog input signal and an error component.
 3. Anadaptive control unit according to claim 2, wherein said gain adjustmentmeans is adapted to use said analog input signal after filtering by saidfiltering means.
 4. An adaptive control unit according to claim 2,wherein said gain adjustment means is adapted to use said analog inputsignal to provide said error component.
 5. An adaptive control unitaccording to claim 2, wherein said gain adjustment means is adapted touse an error signal input to provide said error component.
 6. Anadaptive control unit according to claim 1, wherein said gain adjustmentmeans is adapted to use a filtered reference signal method.
 7. Anadaptive control unit according to claim 1, including analog subtractormeans for subtracting an output of said filtering means from said analoginput signal input before input to said analog filter.
 8. An adaptivecontrol unit according to claim 1, wherein said filtering means isadapted to filter said analog input signal.
 9. An adaptive control unitaccording to claim 8, wherein said filtering means is adapted to filterusing a model of at least a phase response of a feedback path of saidanalog output signal to said analog input signal.
 10. An adaptivecontrol unit according to claim 8, wherein said filtering means isadapted to filter said analog input signal using a model of at least aphase response of a feedback path of said analog output signal to saidanalog input signal and a model of at least a phase response of saidanalog filter.
 11. An adaptive control unit according to claim 8,including analog subtractor means for subtracting an output of saidfiltering means from said analog input signal input before input to saidanalog filter.
 12. An adaptive control unit according to claim 11,wherein said filtering means is adapted to receive said analog inputsignal for filtering after subtraction of the filtered signal by saidsubtractor means.
 13. An adaptive control unit according to claim 8,wherein said filtering means comprises a digital filter.
 14. An adaptivecontrol unit according to claim 8, wherein said filtering meanscomprises a digital filter and a digital subtractor means for digitallysubtracting a digital representation of said analog output signal from adigital representation of said analog input signal.
 15. An adaptivecontrol unit according to claim 13, wherein said digital filter isadapted to filter using a model of at least a phase response of afeedback path of said analog output signal to said analog input signaland said digital filter is adapted to include a factor in said modelcompensating for a delay caused by analog to digital conversion of saidanalog input signal and digital to analog conversion of a digital resultof digitally filtering.
 16. An adaptive control unit according to claim8, wherein said filtering means includes an analog to digital converterfor receiving and digitising the analog input signal, at least onedigital filter for filtering the digitised input signal, and a digitalto analog converter for converting the filtered digitised input signalto produce an analog compensation signal for compensating for feedbackof said analog output signal in said analog input signal.
 17. Anadaptive control unit according to claim 1, wherein said filtering meansis adapted to filter said analog output signal
 18. An adaptive controlunit according to claim 17, wherein said filtering means is adapted tofilter using a model of at least a phase response of a feedback path ofsaid analog output signal to said analog input signal.
 19. An adaptivecontrol unit according to claim 17, including analog subtractor meansfor subtracting an output of said filtering means from said analog inputsignal input before input to said analog filter.
 20. An adaptive controlunit according to claim 17, wherein said filtering means comprises adigital filter.
 21. An adaptive control unit according to claim 20,wherein said digital filter is adapted to include a factor in said modelcompensating for the delay caused by analog to digital conversion ofsaid analog output signal and digital to analog conversion of a digitalresult of digitally filtering.
 22. An adaptive control unit according toclaim 17, wherein said filtering means includes an analog to digitalconverter for receiving and digitising the analog input signal, at leastone digital filter for filtering the digitised input signal, and adigital to analog converter for converting the filtered digitised inputsignal to produce an analog compensation signal for compensating for anyfeedback of said analog output signal in said analog input signal. 23.An adaptive control unit according to claim 1, wherein said gainadjusting means comprises a digital control means.
 24. An adaptivecontrol unit according to claim 23, wherein said digital control meanscomprises a filtered reference control means
 25. An adaptive controlunit according to claim 1, wherein said analog filter comprises anamplifier.
 26. An adaptive control unit according to claim 25, whereinsaid amplifier comprises a digitally controlled amplifier.
 27. Anadaptive control unit according to claim 1, wherein said analog filtercomprises an analog compensation filter.
 28. An adaptive control unitaccording to claim 1, wherein said filtering means and said gainadjusting means comprises a programmed digital controller.
 29. Anadaptive control unit according to claim 1, wherein said filtering meanscomprises a one bit analog to digital converter for converting theanalog signal to a one bit digital signal, a digital filter comprising amodel of the feedback path of said analog output signal to said analoginput signal and adapted to digitally filter the one bit digital signalby a series of additions, and a one bit digital to analog converter forconverting the output of said digital filter to an analog filter outputfor use in the compensation for said fed back component in said analoginput signal.
 30. An adaptive control unit according to claim 1,including a digital filter unit for digitally filtering said analoginput signal; and a combiner means for combining the output of saidanalog filter and the output of said digital filter unit to provide saidanalog output signal for said control unit.
 31. An adaptive control unitaccording to claim 30, wherein said digital filter unit is an adaptivedigital filter.
 32. An adaptive control unit according to claim 30,wherein said digital filter unit is controllable to be adapted using afiltered reference method.
 33. An adaptive control unit according toclaim 31, wherein said adaptive digital filter includes feedbackcompensation for compensating for a fed back component from an output ofsaid digital adaptive filter in said analog input signal.
 34. Anadaptive control unit according to claim 1, for receiving a plurality ofsaid analog input signals each containing at least an indication of theparameter to be controlled to generate a plurality of said analog outputsignals for control of the parameter, wherein each said analog inputsignal contains a fed back component resulting from each said analogoutput signal, the adaptive control unit further comprising a pluralityof said analog filters, each having an adjustable gain; wherein saidgain adjusting means is adapted to adjust the gain of each said analogfilter using a feed forward adjustment method; and said filtering meansis adapted to compensate for said fed back component in each said analoginput signal.
 35. An adaptive control unit according to claim 34,wherein said gain adjusting means is adapted to use a multi-channelfiltered reference method for adjustment of said gain for each saidanalog filter.
 36. A control system for controlling a parameter of aplant, the control system comprising the adaptive control unit accordingto any preceding claim, plant parameter detection means for detectingsaid parameter and for providing the detections as said analog inputsignal, and plant control means for receiving said analog output signaland for controlling said parameter using said analog output signal. 37.A control system according to claim 36, wherein said plant parameterdetection means is adapted to also provide a detected error component insaid analog input signal and said gain adjustment means is adapted todetermine said gain adjustment using said indication of said parameterand said error component.
 38. A control system according to claim 36,including error detection means for detecting an error component,wherein said gain adjustment means is adapted to determine said gainadjustment using said indication of said parameter and said errorcomponent.
 39. An active vibration control system for controllingvibrations comprising the control system according to claim 36, whereinsaid plant parameter detection means comprises at least one vibrationsensor, and said plant control means comprises at least one vibrationactuator.
 40. An active vibration control system for controllingvibrations comprising the control system according to claim 38, whereinsaid plant parameter detection means comprises at least one vibrationsensor, said plant control means comprises at least one vibrationactuator, and said error detection means comprises at least onevibration sensor.
 41. A PID controller comprising: a proportional unitincluding a first control unit for adjusting the gain of saidproportional unit, the first control unit comprising: a first analogfilter having an adjustable gain; first gain adjusting means foradjusting the gain of said first analog filter; and first filteringmeans for compensating for a fed back component in a first analog inputsignal; an integral unit including a second control unit for adjustingthe gain of said integral unit, the second control unit comprising: asecond analog filter having an adjustable gain; second gain adjustingmeans for adjusting the gain of said second analog filter; and secondfiltering means for compensating for a fed back component in a secondanalog input signal; a differential unit including a third control unitfor adjusting the gain of said differential unit, the third control unitcomprising: a third analog filter having an adjustable gain; third gainadjusting means for adjusting the gain of said third analog filter; andthird filtering means for compensating for a fed back component in athird analog input signal; and summing means for summing the outputs ofsaid proportional unit, said integral unit, and said differential unitto generate an output of said PID controller; wherein said first, secondand third gain adjustment means are adapted to determine a gainadjustment using a feed forward adjustment method.
 42. An adaptivecontrol unit for receiving an analog input signal containing at least anindication of a parameter to be controlled to generate an analog outputsignal for control of the parameter, wherein said analog input signalcontains a fed back component resulting from said analog output signal,the adaptive control unit comprising: an analog input for receiving saidanalog input signal and an analog output for outputting said analogoutput signal; an analog filter having an adjustable gain and connectedbetween said analog input and said analog output; a gain control unitconnected to said analog filter for controlling the gain of said analogfilter using a feed forward adjustment method; and a feedback filterconnected to the input of said analog filter for compensating for saidfed back component in said analog input signal.
 43. A PID controllercomprising: a proportional unit including a first control unit foradjusting the gain of said proportional unit, the first control unitcomprising: a first analog input for receiving a first analog inputsignal and a first analog output for outputting a first analog outputsignal; a first analog filter having an adjustable gain and connectedbetween said first analog input and said first analog output; a firstgain control unit connected to said first analog filter for controllingthe gain of said first analog filter using a feed forward adjustmentmethod; and a first feedback filter connected to the input of said firstanalog filter for compensating for a fed back component in said firstanalog input signal; an integral unit including a second control unitfor adjusting the gain of said integral unit, the second control unitcomprising: a second analog input for receiving a second analog inputsignal and a second analog output for outputting a second analog outputsignal; a second analog filter having an adjustable gain and connectedbetween said second analog input and said second analog output; a secondgain control unit connected to said second analog filter for controllingthe gain of said second analog filter using a feed forward adjustmentmethod; and a second feedback filter connected to the input of saidsecond analog filter for compensating for a fed back component in saidsecond analog input signal; a differential unit including a thirdcontrol unit for adjusting the gain of said differential unit, the thirdcontrol unit comprising: a third analog input for receiving a thirdanalog input signal and a third analog output for outputting a thirdanalog output signal; a third analog filter having an adjustable gainand connected between said third analog input and said third analogoutput; a third gain control unit connected to said third analog filterfor controlling the gain of said third analog filter using a feedforward adjustment method; and a third feedback filter connected to theinput of said third analog filter for compensating for a fed backcomponent in said third analog input signal; and summing means forsumming the outputs of said proportional unit, said integral unit, andsaid differential unit to generate an output of said PID controller.